Current procedures for forming a track in a patient involve first piercing the flank of the patient and, for example, the kidney with a small needle for initial access. Proper imaging verifies the correct placement. The track is then opened to a desired size by any of a variety of methods.
In a first method, a balloon catheter, back loaded with a sheath, is inflated to an appropriate pressure. This pressure provides the radial force to dilate the track. After the track is dilated, a sheath is passed over the balloon to maintain the track opening. For different track sizes, an array of balloons of varying sizes are needed because the maximum dilation force is obtained only when the balloon is fully inflated. For each different size balloon catheter, a different sized sheath must be used. This method of dilation, however, as opposed to the second method of renal dilators and sheaths, provides equal radial force around the circumference of the balloon and along its length (so called “continuous dilation”). It is generally preferred for this reason and because it minimizes trauma to the patient. However, balloons are prone to inflate with irregularities when the resistance of the surrounding tissues is irregular such as encountered with segments of scar tissues. The weakest area of the tissue is dilated faster forming a waist, the remaining tissue is dilated as the inflation pressure is increased, but at a slower rate. This phenomenon is referred to as “waisting.”
A second currently used method of dilation involves the use of several passive renal dilator and sheaths. The renal dilator shears tissue as it is introduced into the track. The passive dilator is a plastic tube with a bullet tip; the passive sheath is a hollow tube. Both dilators and sheaths have specific preset diameters. For example, the size of renal dilators range in diameter from 8F to 30F in increments of about 2F (F refers to “French,” a unit of measurement of diameter, where 1F=0.013 inch). An 8F dilator is first used to dilate the track, followed by substitution of incrementally larger dilators until the desired track size is obtained. Thus, multiple exchanges of dilators are necessary. This method causes undesirable shearing of tissue.
In a third method, an elongate tube made of elastic material such as rubber is mounted and stretched longitudinally onto a rod, thus decreasing the radial size of the tube. The mounted tube is inserted into a track opening and released from the rod, thus increasing the radial size of the tube to its original size. Using this system, one needs a series of tubes having varying radial sizes to fit the desired track opening. Selection of the desired size tube permits the dilation of a track opening in one step. However once a tube is selected and positioned, the size of the track cannot be modified.
In a fourth method, the dilators have a tapered distal end formed by a compacted slit tube or a series of foil wrapped onto one another to form a cone which may be gradually expanded by manual insertion of a rod within the interior of the dilator until the wall of the dilator's distal end have become parallel.
In yet another method, a mesh tube is introduced percutaneously in the body using a needle and stylet. Once placed into the body, the needle and the stylet are removed from the mesh and a passive dilator is inserted manually to expand the radius of the mesh tube. Using this system, still several passive dilators are needed to gradually reached the desired track size.
A major inconvenience of these methods is that for each desired track diameter, one must use a specific dilator or balloon and a specific sheath. The limited availability of sizes for these components limits the availability of track sizes that maybe obtained. Further, once these components are inserted, the track size cannot be constricted.